The backbone of virtually all electronic systems is the printed circuit board which serves as the primary mechanism for mounting as well as interconnecting electronic components. Conventional single-layer printed circuit boards are typically manufactured by first laminating a thin sheet of copper to a resin substrate such as FR-4 or the like. The copper is then photolithographically patterned to yield the desired arrangement of copper circuits (traces) on the substrate surface. Through-holes may be drilled to create apertures for receiving component leads. Once the holes have been drilled, the substrate may then be electroless copper plated to metallize the through-holes to yield a single layer circuit board. Multi-layer circuits are produced by laminating very thin individual single-layer boards together. The copper traces on each layer of a multi-layer board are selectively connected to those on one or more adjacent layers by way of metal-plated through-holes (vias).
Careful control of the thickness of the copper traces on the circuit board is extremely important. If the copper traces are too thin, then the impedance of the trace may become too large, adversely affecting the circuit board operation. In addition, traces which are too thin may prematurely crack or break. If the copper traces are too thick, then difficulty may be incurred in laminating successive layers of the multi-layer board together. In addition, very thick copper traces are wasteful, increasing the cost of circuit board production. For these reasons, it is useful to monitor the copper trace thickness.
In the past, conventional optical thickness measurement techniques such as triangulation, have not proven themselves successful for accurately measuring the copper trace thickness on the circuit board. The reason is that such techniques usually require the board area adjacent to the trace to reflect light into a light-sensing mechanism, such as a photodiode or the like. Typically, FR-4 material is light translucent, making it extremely difficult to obtain light reflection from the top surface of the board.
Because of the inability to use conventional optical thickness measuring techniques, the thickness of the copper traces on a circuit board is presently established by physically removing the copper traces, typically by peeling them from the board, and the measuring their thickness using conventional methods. While this technique is very effective, yielding very accurate results, the board is destroyed during the process. Consequently, it is only possible to measure the copper trace thickness on a sampled basis.
Thus, there is a need for a technique for non-destructively measuring the thickness of a light-reflective layer (e.g., copper trace) on a light-translucent substrate (e.g., a circuit board).